The present invention relates to a method for fine-machining of workpiece surfaces to be supplied with a lubricant during operation of the workpiece, especially for fine-machining of bore walls in cylinders of an internal combustion engine, whereby the workpiece surface is provided with linearly extending, especially crossed (intersecting), recesses.
For fine-machining of workpiece surfaces honing processes are known which are generally used in order to provide a bore wall, serving as the piston guide surface in a cylinder of an internal combustion engine, with the finished dimension. Conventionally, a plurality of honing operations are performed in order to reach the finished dimension. For the honing process honing stones are commonly used which are moved back and forth into the bore with axial stroke and simultaneous rotation about the bore axis whereby in the surface to be machined crossed (intersecting) honing traces are generated which during the later use of the workpiece that, for example, is comprised of cast-iron, provide a reservoir for the required lubricant. For reaching the finished dimension it is also possible to use a plateau honing process as a last method step in which with a very fine cutting tool only the peaks of the already honed surface are removed. With this process, the supporting guiding surface for the piston can be enlarged during finishing honing of the piston guide surface.
It is also known to use a last method step during fine-machining of bore surfaces in which the workpiece surface is subjected to a stream of a medium, whereby preferably laser beams are used (German Offenlegungsschrift 39 32 328). With the laser beam the peaks of the honed surface are removed primarily by vaporization whereby flakes and loosely attached particles are also removed. The honing traces remain intact with this method of machining.
For finishing the finish-honed bores it is furthermore known to remove flakes by brushing and/or high pressure jets and to expose thereby graphite lamellas of the cast iron material (German Offenlegungsschrift 37 19 796). In this method the honing traces also remain intact (they are required for the subsequent lubricant distribution over the entire honed surface).
The honing traces which result from the machining by honing with the honing stones are irregular with respect to width, depth, and distance corresponding to the material-based irregularities of the cutting surface of the honing tool. The profile of the honed surface therefore consists of a conglomerate of greater and smaller peaks and valleys which, in the case of plateau honing, also consists of flattened areas. Thus, in the mass production of engines considerable differences result in the operation of the individual engines.
For improving the lubricant distribution of the fine-machined surface it has been suggested, especially for piston guide surfaces in cylinders, to carry out a method step of subjecting, after the honing step resulting in only a slight roughing depth, the surface to a stream of material with which grooves of a greater depth can be generated according to a predetermined design whereby the step of subjecting to a stream of material is performed either with a laser beam or with a liquid stream under high pressure (European document 92 106 305). In this process grooves of a predetermined depth are produced according to a defined design, for example, with uniform crossings (intersections) whereby the material at the edge of the grooves is not deformed in any way, shape or form. The grooves thus provide a lubricant reservoir with a uniform distribution of the lubricant.
It is therefore an object of the present invention to provide a substantially precisely predetermined surface structure by a fine-machining step which surface structure can be adapted to the subsequent operational conditions of the workpiece in an optimal manner.